1. Field of the Invention
The present invention relates to ceramic compositions for multi-layer ceramic capacitors, and more particularly to ceramic compositions having a high dielectric constant making it possible to produce miniature multi-layer ceramic capacitors.
2. Description of the Prior Art
High dielectric ceramic compositions with barium titanate (BaTiO.sub.3) as main component have been used in making multi-layer ceramic capacitors. Conventionally, many proposals have been made which involve addition of various additives for improving unsatisfactory characteristics. However, they have a difficulty in determining the effect of one additive or the effects of several additives on the overall characteristics by varying the amount of one additive or simultaneously varying the amounts of several additives. As a result, it is difficult to obtain a composition capable of exhibiting desired stable dielectric constant and insulation resistivity and to find optimum amounts of additives making it possible to achieve the same characteristics.
There have been also proposed dielectric compositions capable of satisfying the characteristic of X7R of EIA (Electric Industries Association) standard. The X7R characteristic means that the change rate of dielectric constant measured at a temperature range of -55.degree. C. to +125.degree. C., taking 25.degree. C. as reference is not more than .+-.15%. The dielectric compositions are barium titanate-niobium oxide (BaTiO.sub.3 -Nb.sub.2 O.sub.5) based compositions including additives. Japanese Patent Publication No. 61-99207 discloses a high dielectric ceramic composition including MgO and CeO.sub.2 as additives. Japanese Patent Publication No. 62-229605 discloses the use of Co.sub.2 O.sub.3, Mno.sub.2 and CeO.sub.2 as additives. On the other hand, Japanese Patent Publication No. 60-19606 discloses a high dielectric ceramic composition including CaO and ZnO as additives. Although exhibiting high dielectric constant of 3,000 or above, however, these compositions disclosed in the publications show high loss factor (tan .delta.) of not more than 0.8% measured at a frequency of 1 KHz and low insulation resistivity of 10.sup.11. They also show unstable temperature dependant capacitance change rate (TCC) which varies irregularly depending on variation in composition.